Reprint from „MPT International“ Volume 26 (2004), issue No. 4
Copyright. Verlag Stahleisen GmbH, D-Düsseldorf
Self-lubricating, maintenancefree sliding materials
The use of self-lubricating sliding materials is gaining steadily
in importance in all areas of
production in the steel industry.
Particularly against the background of constantly increasing
competitive pressures, plant
and equipment makers and
operators are today calling for
solutions that offer technical
and also economic benefits in
equal measure. Sliding bearings
and guides that can do without
lubrication, or critical applications which cause problems
even when additional lubrication is available, consequently
represent an enormous potential for optimisation and savings.
Figure 1. Thick-walled sliding bearings, Deva-Metal type
Hubert Hilp, Application Manager Steel,
Federal-Mogul Deva GmbH, Stadtallendorf,
Germany
Far-reaching changes that have
taken place in the steel industry in
past years caused an increased interest
in maintenance-free sliding materials.
The heavy workforce cut-backs of
recent years especially present problems when it comes to carrying out
the (lubrication) activities that are
required, for example, while the
steadily rising expense of disposing of
spent greases offers a further incentive
for doing without lubrication where
possible. There are, for these reasons,
continuous efforts in the steel industry
to change over as many bearings and
guides as possible to maintenance-free
sliding materials.
Thyssen Krupp Stahl AG, Germany,
for instance, which has taken on a pioneering role in this regard and has its
own tribological department to deal
with the topic of "cost reduction
through freedom from maintenance",
also cites the more stringent requirements regarding plant and equipment
availability, occupational safety and,
not least of all, product quality as the
reasons for the greater use of self-lubricating materials.
Thick-walled sliding bearings
with microscopically distributed solid lubricant
Various sliding materials are
designed for different fields of use. The
Deva-Metal product encompasses a
group of composite materials based on
three different families of alloys,
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namely, bronze, iron and nickel. They
contain solid lubricant, which is uniformly distributed and embedded in
the metal structure (figure 1).
The composition of the metallic
structure determines the physical,
mechanical and chemical properties
of an alloy and therefore serves as a
basis when a material is to be selected
for a special application. The main
criterion is the temperature at the
bearing location.
Bronze and lead bronze are very frequently used for temperatures up to
350°C, with the material having to
undergo a special heat treatment for
continuous service at temperatures
from 150°C and above. Iron- and ironnickel-base alloys are used for continuous service at temperatures over
350°C. Applications involving particular requirements with regard to temperature and corrosion conditions
necessitate a nickel-base or corrosionresistant steel sliding material.
The type of solid lubricant, its form
and, above all, its content of the
metallic structure in terms of percentage depend on the operating requirements, e.g. the sliding velocity and
contact pressure. The solid lubricants
therefore have a decisive role to play,
in principle, in the sliding behaviour
of an alloy. Graphite, molybdenum
disulphide, tungsten disulphide and,
increasingly, PTFE are especially used
as solid lubricants in sliding bearing
technology.
Example: tube transfer device. As
shown in figure 2 the equipment
installed by SMS Demag in 1972 at
Vallourec & Mannesmann in Mülheim
serves to transfer tubes from the production line sideways onto a rake-type
cooling bed. In order to avoid major
deformations of the still hot tubes, the
height of fall in-between is surmounted in two steps. First of all, the tubes
fall into an angular repository, from
where they are then lifted in a second
step by pusher devices and roll onto
the rake-type cooling bed.
The lever mechanism beneath the
pusher devices that initiates the necessary motion sequences was fitted with
pure graphite sliding bearings to start
with. In 1983 the maintenance department changed the bearings over to
metallic sliding bearings with embedded solid lubricant, which tripled the
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Figure 2.
Tube transfer
station at Vallourec & Mannesmann, Mülheim, Germany
Figure 3.
Thin-walled sliding bearings,
Deva-BM type
service life of the most heavily stressed
sliding bearings in the forward area of
the cooling bed. The new bearings
achieve a service life of approximately
2 years despite temperatures of about
200°C, a slewing motion through 45°
and around 5700 movements per day.
The bearings at the rear of the cooling
bed have not as yet been replaced.
This example is one of several identical applications around the world. At
the present time, projects and test runs
are in progress in Canada and Italy, for
instance, where despite permanent
lubrication of the conventional bronze
sliding bearings, plant and equipment
operators are failing to achieve satisfactory service life spans. The sliding
bearings are replaced in either case
some 3 times per year.
Given the experience available, it can
be assumed that the life span can be
prolonged considerably and the maintenance costs greatly reduced as a result.
Thin-walled sliding bearings
with microscopically distributed sliding material
The Deva-BM bimetallic composite
is produced in a special rolling and sintering process. It comprises a steel
backing and a self-lubricating bronze
sliding layer with a microscopically
distributed solid lubricant (figure 3).
Besides high stability under load, thinwalled sliding materials combine the
tribological properties of thick-walled
bronze with compact design.
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Figure 4.
Ladle tilting
stand at the Hüttenwerke KruppMannesmann
works in Duisburg, Germany
Particularly worthy of mention here
is the increasing use of PTFE as a solid
lubricant, which utilises the acknowledged good sliding characteristics of
this material and combines them with
the wear resistance of a metallic
matrix.
One aim is to avoid the undesirable
stick-slip effect, often referred to in
everyday language as chatter or backslip. The stick-slip effect is a phenomenon that occurs with unfavourable
mating surfaces, which, as a result of
their movement in relation to one
another, alternate between sliding and
static friction.
stick-slip effect. In addition, the
changeover has led to even better utilisation of the material because, given
the permissible wear of 1.5 mm, the
first design would have meant that
28.5 mm of the material thickness
would not have been used.
The ladle tilting stand, which was
put into service in 1996, was fitted
with new sliding plates in the summer
of 2002, corresponding to a service life
of around 6 years.
Example: ladle tilting stand. The
special ladle tilting stand structure at
Hüttenwerke
Krupp-Mannesmann
(HKM) in Duisburg, Germany, is an
illustration of good cooperation
between the plant and equipment
operator (facility planning department), plant and equipment maker
(design) and sliding bearing manufacturer (figure 4).
Initially, the use of 30 mm thick
bronze sliding plates with macroscopically distributed solid lubricant was
planned for the structure. The customer's requirement for stick-slip-free
movement, and maximum permissible wear of 1.5 mm, led to the use of
thin-walled sliding plates with a selflubricating sliding layer. In view of the
general on-site conditions, the sliding
layer was additionally provided with
cleaning grooves to reduce the abrasive effect of scale deposits. The sliding
plates were underlaid with steel plates
for height equalisation purposes.
The benefit for the customer has
been, first of all, the avoidance of the
Deva-Tex, a self-lubricating, glassfibre-reinforced composite shown in
figure 5, is manufactured using a special winding technology. The carrying
layer makes the high strength levels
possible, while the sliding layer contains special fibres and solid lubricant
that provide the tribological properties.
Composite sliding bearings
made of glass-fibre plastics
Figure 5. Composite sliding bearings,
Deva-Tex type
In comparison with metallic materials, glass-fibre-reinforced plastics are
clearly limited when applied at temperatures above 150°C and in very
abrasive environments. The plastics
display their advantages, though,
when it comes to applications involving high shock loads or heavy edge
loading.
Use in cranes and braking systems.
Hardened bushings or also metallic
special-purpose materials have been
replaced with plastics-based sliding
bearings in diverse crane systems, for
example. The reason for this has
chiefly been that metallic bushings
have not withstood the high shock
loads and in some cases have been
completely destroyed.
Slab brakes are special constructions with which slabs approaching
on a roller table are decelerated by
means of a lever protruding between
the rollers. A plastic has already been
used successfully for original equipment in expectation of a high contact
pressure exerted by the lever combined with the shock loading of the
bearings.
Dry running tribological system
The principle according to which all
the materials described here are also
able to operate without the presence of
conventional lubricants is basically
the same. The movement taking place
in the bearings generates continuous
micro-abrasion, thereby releasing the
solid lubricant, compressed and
embedded in the sliding material,
from the sliding layer. A durable lubricant film forms as a result on the mating material. Because of their chemical
structure, the solid lubricants are able
to separate the two mating surfaces
permanently.
The wearing of this sliding film, e.g.
due to high frequency of movement,
scale, dust, etc., leads, through the
increasing friction, to the release of
lubricant and hence to the renewal of
the lubricant film, resulting in maintenance-free self-lubrication.
In comparison with lubricated sliding
bearings, the benefits of this functional
principal become evident particularly
when giving closer consideration to the
critical operating conditions within a
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Operating conditions
Effects on the sliding bearings
High temperature
Special lubricants necessary,
lubricants can decompose,
lubricants resinify in lengthy lines,
increased consumption possible
Edge loads
Local increase in contact pressure,
lubricant film may be interrupted,
increased wear
High contact pressure
Lubricant film is interrupted,
lubricant film is pressed from the bearing,
increased wear
Dirt
Adsorption of dirt in the bearing,
formation of an abrasive paste,
constriction of diametral clearance,
possible jamming of shaft
Angular movements
No lubricant reaches the actual load zone
Lengthy downperiods
Lubricant is squeezed from the load zone
Table 1. Effects of difficult operating conditions on sliding bearings
steel plant. Table 1 shows some of these
conditions and indicates which have
effects on the bearings.
Outlook
A recapitulative look at the abundance of uses for self-lubricating
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materials that have already established themselves in the steel industry over the long term, such as in
flame-cutting machines, presses,
sheet levellers, tube straighteners,
ladle turrets, cooling beds, reel mandrels, heat-treating furnaces, finishing equipment, roll-changing systems, and guide systems, clearly illus-
trates the broad spectrum of applications.
The question as to when the use of
appropriate materials is technically
possible and economically practical
has to be clarified individually, which
is why particularly close cooperation
between the user and manufacturer is
necessary beforehand.
This offers the opportunity for adequate consideration to be given to all
the operating conditions when selecting the material or mechanical construction and for finding the best
solution both technically and economically.
An overall look at all the costs
involved, from the procurement costs,
to the costs of lubricant consumption,
disposal, manpower assignment,
repair and lost production, through to
scrap costs, reveals what financial savings potentials there are and whether
they justify the use of maintenancefree sliding bearings. Should the mentioned competitive pressures on the
market intensify further, and that is to
be expected, then this will call for
innovative solutions.
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